In the restoration and reconstruction of a tooth, the installation of permanent crowns is a commonplace procedure. In those cases where the tooth is not severely decayed or otherwise mutilated, the tooth may be prepared to remove the decayed or damaged portions thereof and to provide a "stump" which is adapted to receive the crown, jacket crown or cap. However, in those cases where the tooth is decayed or mutilated to such an extent that a major portion of the natural clinical crown, i.e., the portion which extends above the gum line, is lost leaving only the root structure and a portion of the clinical crown intact, the tooth cannot be prepared to provide a sufficient stump suitable to receive the crown jacket. Until recent times, the severely decayed or mutilated tooth was usually extracted and subsequently replaced with either a fixed or removable bridge as required for aesthetic and/or functional purposes.
With the advent of greater patient acceptance of root canal treatment (endodontia) and the development of new devices, materials and techniques, restoration and reconstruction of even such a badly decayed or mutilated tooth is now possible. Known restoration and reconstruction techniques generally require endodontia, the use of a post fitting within the voided root canal and a hard core integrated with the post to receive the crown. Such posts usually occupy the upper two-thirds of the canal and the core is usually located in the pulp chamber as well as in a portion of the stub of the tooth. The core, which essentially forms an artificial dentine stump, need be built only high enough to mount and reinforce the final crown, jacket crown or cap.
According to one known method for restoration and reconstruction of a badly decayed or mutilated tooth, namely the "Post and Core" method, the core is built up on a plastic or metal post with wax, drop by drop, to the approximate size of the tooth and is then shaped to the desired form with an instrument. Alternatively, the core may be built up by brushing on the post in layers an acrylic plastic which is then shaped to the desired form. In either case, the core may be built up inside the mouth directly on the prepared tooth stub (the direct technique) or outside the mouth on an impression casting (the indirect technique). Regardless of the specific procedure followed, a wax or plastic pattern results which is separated from the prepared tooth stub or impression casting. A casting is then made by the well known "lost-wax" technique to produce a cast metallic post and core which is then cemented in place into the tooth. This method, however, is extremely time-consuming for the patient and dentist and, if the direct technique is followed, uncomfortable for the patient.
Another method heretofore known, namely the "Composite Resin Build-Up" method, uses a precious or non-precious metal post cemented into the root canal, optional anchor pins screwed into the tooth stub, and a core built up on the post and pins by employing a plastic resin composition. After the root canal therapy is completed, the post, and optionally the pins, are secured in the stub. If used, the pins are normally provided with threaded or knurled surfaces adapted to interlock with the mass of air-hardenable platic resin composition retained in position against the stub by means of a core form or matrix pending hardening. Following hardening and removal of the core form, the mass of hardened resin composition is ground to the desired size and shape for receiving the final crown, jacket crown or cap.
This "Composite Resin Build-Up" method also has the drawback of being time-consuming and further requires removal of large amounts of the hardened plastic composition to produce a core of the desired size and shape as the core forms generally used today are cylindrical whereas a truncated conical core is generally preferred. Furthermore, the bulk of restorative material present as dictated by the size of the form required may be in close relationship with adjacent teeth whereby the latter may be injured unnecessarily by dental burrs or diamonds when the hardened plastic composition is being ground and shaped into the final preparation form. Moreover, a relatively large stock of forms of different sizes must be stocked by the dentist to accommodate the various tooth sizes.
One of the more common core forms employed to carry out the "Composite Resin Build-Up" method is a cylindrical copper band which is supplied in various sizes. However, such bands are not ideally suited for use in the "Composite Resin Build-Up" method in that fitting a copper band to a tooth is both difficult and time-consuming. Moreover, a very closely adapted copper band must be used in the "Composite Resin Build-Up" method because the composite resin material is relatively free flowing when initially mixed, and when placed on the tooth stub, any void between the tooth and copper band would allow the material to extrude beyond the copper band and the proposed preparation termination while held under finger pressure. Because the composite resin adheres tenaciously to natural tooth structure at times, such excess material is very undesirable since some small portions beyond the limits of the final preparation form may go undetected and later on act as a chronic irritant to the surrounding supportive tissues. Also, during the initial process of placing the composite resin filled copper band on the tooth and holding the same under firm finger pressure, because the copper band has a sleeve fit interrelationship to the remaining tooth structure, it is quite possible to over-seat the band beyond the desired point and thus cause injury to the surrounding gum tissue or to the peridontal attachment which supports the tooth. Because the copper band usually is cut by scissors at the gingival aspect thereof to mimic the cervical line of the tooth, the sharp cut edge of the band further enhances the possibility of such injury. Removal of the copper band may also be difficult and harmful to the patient as such removal is usually accomplished by cutting a continuous vertical slot through the copper band, spreading the band, grasping the band with a small forceps or similar instrument and then withdrawing the same from the tooth. During the actual removal procedure, the sharp edges of the cut copper band often cause injury to the surrounding oral tissue.
Still another known method for restoring a badly decayed or mutilated tooth utilizes a core form of transparent plastic that may be made available to the dentist in a range of sizes calculated to fit closely on the tooth stub, depending upon its size. Such form comprises a cylindrical lateral wall and an end wall fabricated of a transparent, thin, but relatively rigid, plastic material having a good memory. A form of a proper size for the tooth is located over the tooth stub and a previously positioned post and is festooned to conform to the irregular surface of the tooth stub. After being festooned, the device is inverted and filled with wax, self-curing plastic or other material and when no longer in a free flowing state, it is then forced over the tooth stub with the excess thereof being squeezed out at the lower margin of the side wall and immediately trimmed away. When the plastic material hardens, the form is cut and stripped from the core, after which the core and post pattern are separated from the stub and a casting is made by the well-known "lost-wax" technique and later cemented into the tooth stub.